Flood mapping
Flood maps decide insurance requirements and constrain building. They are also drawn at a scale that cannot see your individual lot, which is where the useful opportunity sits.
FEMA publishes Flood Insurance Rate Maps showing special flood hazard areas, the zones with a one percent annual chance of flooding, commonly called the hundred year floodplain.
If your structure sits in one and you have a federally backed mortgage, flood insurance is mandatory. Local floodplain regulations also apply to building and substantial improvements within the mapped area.
These maps are produced at a scale that works for regional planning. They cannot resolve individual parcels, and they do not attempt to.
The practical consequence is that a house sitting on a knoll several feet above the surrounding ground can be inside a broadly mapped zone purely because the mapping was not fine enough to exclude it. The map is not wrong at the scale it was drawn. It is just not answering the question you have about your specific building.
An elevation certificate establishes the actual elevation of your structure against a published vertical datum, and compares it to the base flood elevation for your location. That converts a regional map into a specific fact about your building.
If the survey shows your structure sits above the base flood elevation, you can apply to FEMA for a Letter of Map Amendment. A successful LOMA removes the mandatory insurance requirement for that structure.
On a property carrying a substantial annual premium, the survey frequently pays for itself in the first year or two. It is worth checking whenever the house sits noticeably higher than the ground around it.
Maps change. FEMA updates flood mapping periodically. A property outside a zone a decade ago can be inside one now, and the reverse happens too. If your last elevation certificate is old, it reflects a map that may no longer be in effect.
Standard flood modeling assumes water flows across the surface and collects in identifiable channels. On the Springfield Plateau a meaningful amount of drainage goes underground through sinkholes and losing streams.
That cuts both ways. Some areas drain better than surface modeling suggests. Others flood in ways the mapping does not anticipate, particularly where a sinkhole that normally swallows runoff backs up during a large event. Localized flooding around karst features is a real phenomenon here and it does not always correspond to a mapped zone.
See sinkholes and your property.